(a) Field of the Invention
The present invention relates to light-emitting display, and more particularly it pertains to light-responsive light-emitting diode display to be used in such places where ambient brightness varies largely.
(b) Description of the Prior Art
Many of the conventional outdoors-installed colored-light display devices (hereinafter to be referred simply to simply as display devices) have employed a combination of an incandescent lamps and colored glass or plastic plates, or gas-filled discharge tubes which emit colored-lights. Traffic signal lamps are typical examples of the former, and neon signs are typical examples of the latter. Ordinary gas-filled discharge tubes have weak light outputs and it is difficult for the viewer to obtain clear recognition of such lights under daytime sunlight. Besides, gas-filled discharge tubes have limited service lifetime. These are considered to be the essential drawbacks of the gas-filled discharge tubes, and no striking improvement in their ability can be expected. Accordingly, combinations of incandescent lamps and filters such as colored glass plates have generally been used for display devices as traffic signals which are required to perform display operations outdoors day and night.
Fluorescent lamps have a higher electric power-to-light conversion efficiency than do incandescent lamps. However, the former have such drawbacks as larger size, lower brightness and slower lighting-up action as compared with incandescent lamps, as well as the inconvenience of requiring a power supply source for their exclusive use. It is for these reasons that fluorescent lamps are typically not used for many purposes other than for providing continuous illumination.
Accordingly, incandescent lamps are employed in many of those colored-light displays designed to perform alternating displays within a limited area on the display device, as in traffic signal devices.
In order to provide a distinctive colored-light display in places where ambient brightness undergoes substantial changes, it is necessary that the light-output always be capable of coping with the ambient brightness, where an incandescent lamp is used, the light-output is determined mainly by the temperature and the surface area of the light-emitting portion (filament). Therefore, if the supply electric voltage is reduced, this will cause a drop in the temperature of the filament, and will bring about a sudden drop in the light-output or a change in color temperature. Thus, it is not easy to adequately control the light-output of an incandescent lamp in accordance with the changes in the ambient brightness. If the light-output is fixed to provide a fixed constant brightness, it is necessary to design the display device provide a distinctive display when the ambient light is at its maximum brightness. By so arranging the display device, however, the device will naturally perform display which will greatly exceeds the brightness which is required when the ambient light has become dark. Such unnecessary consumption of electric power would be tremendous even when traffic signal lamps alone are considered. Thus, improvement of such devices of the prior art has been desired and demanded not only by the society in general but also by the users of display devices.
As such, an improvement in the electric power-to-light conversion efficiency of colored-light display devices is desired, as well as an adequate and easy control of the amount of emitting lights.
Semiconductor light-emitting diodes have made a remarkable progress in their efficiency. The intensity of their emitting lights have now become to be substantially proportional to the value of the current flowing therethrough, and bright displays can be provided even by low-current operations. Moreover, light-emitting diodes have an average service lifetime of much longer than 100,000 hours. As such, it is very desirable that almost all those colored-light display devices which are presently being used outdoors be replaced by those display devices using light-emitting diodes from the viewpoints of energy-saving and long service life.
Most of the colored-light displays can be performed with the three colors, i.e. red, yellow and green (blue). High-efficiency light-emitting diodes emitting red, yellow and green colors, respectively, can be manufactured by arranging their light-emitting regions with, for example, Ga.sub.0.7 Al.sub.0.3 As, GaAs.sub.0.7 P.sub.0.3 or InGaAsP for red color emission, In.sub.0.6 Ga.sub.0.4 P, or Al.sub.0.4 In.sub.0.6 P for yellow color emission, and GaP, or In.sub.0.7 Ga.sub.0.3 P or Al.sub.0.5 In.sub.0.5 P for green color emission, respectively. It is needless to say that light-emitting diodes which are capable of emitting lights having wavelengths different from those of said three colors mentioned above can be manufactured as well. At the present stage, the diodes which emit blue color have a much lower efficiency, but a remarkable hike in their light-emitting efficiency can be expected. It has been already proved by, for example, the so-called temperature difference method under controlled vapor pressure (TDM-CVP) that a light-emitting diode can be obtained having a very high efficiency if the compositional ratio of the respective regions of a light-emitting diode having a multi-layer hetero-junction structure is held under strict control and if lattice alignment between the respective adjacent regions is performed to grow crystals having few defects. Techniques have advanced so far that the colors of lights emitted from light-emitting diodes cover almost the entire visible range. Therefore, it is possible to select of any arbitrary combination of these diodes to suit a given purpose.
Even when those colored light display devices using incandescent lamps as their light sources are replaced by display devices using light-emitting diodes, and the maintenance of the display devices has thus become simplified and the electric power consumption greatly reduced, there still remains the problem concerning the adjustment of intensity of light of the display devices for such place where ambient brightness varies largely. That is, operating the device at the same intensity, when the ambient light has become dark, as when ambient light is at its maximum brightness, will use electric power several times that which is really required for that period when the ambient light is much darker than the daytime.